There is a fast growing commercial demand for monitoring systems capable of identifying and quantifying constituent gasses in a sample. The demand arises from the need to insure the purity of substances during the production process, and from the need to prevent hazardous gasses from escaping into the air. Monitoring in the latter context is becoming even more urgent as governments worldwide enact clean air regulations which limit the emissions of hazardous gasses from processing plants and storage facilities. The regulations compel the use of a monitor to enable corrective action in case the concentration of hazardous gas exceeds a specified level, and to provide an early warning of impending danger to plant personnel and the public.
Efforts to satisfy the commercial demand have resulted in a wide variety of monitors each dedicated to sensing different gasses in different environments. For instance, a frequent need arises to analyze ammonia. Ammonia is a toxic and flammable substance widely used in many manufacturing and chemical processes. In addition, it is a prime ingredient in the widespread process known as de-NOx. More specifically, in any open-air burning, compounds comprising oxides of nitrogen (NOx) are produced. The NOx poses a significant health hazard and contributes to acid rain, hence the emission rate is strictly regulated. To remove the NOx, ammonia is introduced in the exhaust stream, and the stream is passed over a catalyst. The NOx is removed by reaction with the ammonia and non-toxic byproducts are exhausted. Unfortunately, the de-NOx process is extremely difficult to control, and significant amounts of ammonia may escape through the exhaust smokestack.
In addition to de-NOx, unwelcome ammonia may also appear in certain process streams, for example, in the processing of ethylene. In order to purify the ethylene, trace amounts of ammonia concentration must be accurately measured.
Ideally, an ammonia monitor suited for the above-described analysis of ammonia should operate continuously over extended periods of time without the need for frequent maintenance or calibration. Current monitoring systems use electrochemical sensors, spectroscopy, and related devices. These systems are generally inadequate because they lack specificity, require frequent maintenance, calibration, or replenishment of electrolyte. The electrolytic systems are limited to operating at ambient temperatures above 0.degree. C. (due to freezing of the electrolyte).
In contrast, an Ion Mobility Spectrometer (IMS) is a well-known analytical tool capable of accurate and trouble-free analysis of the constituents in a sample. Basically, an IMS comprises an analyzer cell, means for ionizing samples of an analyte admitted to the cell and means for determining the times required for the ions of the various substances present in the cell to traverse a specific length of the cell under the influence of an electric field and against the force of a stream of drift gas flowing through the cell in a direction opposite to that of the electric field. A representative analyzer cell is disclosed in U.S. Pat. No. 4,390,784 issued to Browning, et al. A stream of purified gas may be used as a carrier gas to introduce the analyte sample into the cell, and a stream of purified gas may also be used as the drift gas. If the carrier gas and the drift gas are readily available at an installation site in unlimited quantities, then there is no maintenance required of the sensor other than the occasional replacement of filters and membranes for purifying the carrier and drift gasses, radiation wipe tests and calibration. An IMS is therefore well-suited for use in a monitoring system designed to detect and quantify hazardous gases.
Unfortunately, it has been found that an IMS operated in a conventional manner, using air as the carrier and drift gasses, may lack the specificity necessary to detect ammonia under certain conditions. The conditions arise when interferants are present in the sample. For instance, in the de-NOx process there are hydrocarbons. In processes involving ethylene, the ethylene itself acts as an interferant. This is because interferants (such as hydrocarbons and ethylene) disrupt the ammonia peak. Hence, it becomes very difficult to distinguish the amplitude of the ion current due to the ammonia gas from the ion current due to the other sample constituents.
In application Ser. No. 534,701, now U.S. Pat. No. 5,032,721 issued Jul. 16, 1991 a monitor is disclosed which uses a dopant to improve the specificity for acid gasses. The dopant is selected from the group of substituted phenols, and improves the ability of the gas monitor to detect the general presence of acid gasses such as hydrogen fluoride, hydrogen chloride, chlorine, nitrogen dioxide, sulfur dioxide, carbonyl sulfide, and numerous others. This ability to improve IMS specificity for detection of acid gas is a significant refinement, but it does nothing to improve the specificity of detection for ammonia.
Likewise, the use of a dopant was disclosed in application Ser. No. 687,594 for improving the IMS specificity toward acid gases in air. The higher specificity is achieved by introducing a controlled concentration of sulfur dioxide dopant to the air carrier gas stream. The reaction with the acid gas causes the drift times of the ions generated from the doped air carrier gas to differ from the drift times of the ions generated from the acid gas analyte, thereby allowing identification and quantification of the acid gas analyte. This dopant chemistry is also advantageous for the detection of acid gasses, but cannot improve the specificity of detection for ammonia.